Finding the right size using the metric system is much easier than calculating fractions in inches. These drills were pretty sticky with some residue and they don't look good. But they work well and the price is good. The only problem was that they wobbled because with one hit they are only clamped onto the Allen bit from the side and this changes the center a bit. This isn't a problem unless you're using a drill press and need a dead center. The size mark is on the side of the hex shank for easy identification.